Number 274 (Story #2), June 7, 1996 by Phillip F. Schewe and Ben Stein|
THE PHYSICS OF CAR BATTERIES: Originally intending only to provide a proper explanation of batteries for an undergraduate electromagnetism book he is writing, Wayne Saslow of Texas A&M University (409-845-4841) has uncovered new details of how car batteries work. A 12-volt car battery connects six lead-acid cells end to end. Each cell consists of a lead (Pb) electrode and a lead oxide (PbO2) electrode immersed in a solution of water and sulfuric acid (H2SO4). The acid dissociates into H+ and HS04- ions. When connected to an external circuit, the reactions at the electrodes drive ion current within the cell and electron current in the external circuit. Typical theories of electrochemical cells assume that the amounts of positive and negative charge exactly balance (electroneutrality). However, for a slow, steady discharge of the battery and idealized flat electrodes, Saslow's analysis finds that there is an excess of HSO4- ions in the bulk of the solution. Overall "global" electroneutrality is maintained by an excess of H+ ions near the electrodes. In addition to painting a more sophisticated picture for electrochemical cells, his results may have implications for the study of ion channels in biological cells, and for astrophysical plasmas, in which "local electroneutrality" is conventionally assumed. Saslow's equations also offer insights on how to design a cell which would provide maximum power at a uniform rate until it becomes totally depleted. This would be relevant for the design of electric vehicle power packs. (17 June, Physical Review Letters).